1. Field of the Invention
The present invention relates to underwater buoys designed to be submerged and held inside the water at the end of a cable which is itself hooked to a carrier vehicle such as a helicopter. These buoys can be used inter alia for the detection of submerged objects, notably submarines, either by passive listening or by means of a sonar.
2. Description of the Prior Art
Since submarines are able to descend to increasingly great depths, it is necessary, in order to detect them with certainty, to take detection buoys down to depths of the same order, namely depths of several hundreds of meters. This, of course, makes it necessary to unwind and wind the carrier cable along the same length, while preventing oscillations which are harmful as much through variations in tension given to the cable as through the risk of the coils getting jumbled on the drum of the winch on which this cable is wound.
These oscillations are due to the slowing down of the buoy when it goes askew or even sideways under the effect of phenomena of hydrodynamic instability due to the relative motion of the water with respect to the buoy. To remove this instability and keep the buoy vertical while it descends or rises, there are known ways, as shown in FIG. 1, to provide the body 10 of this buoy, suspended to the end of the cable 11, with an upper ring 12 and a lower ring 13 which surround the ends of this body in setting up, between the body and themselves, a space designed to let through the streams of water while the buoy moves. Furthermore, the buoy has a ballast 14 placed at its lower end.
The effect of these rings differs according to whether the buoy is plunging or rising up again. The top ring 12 is efficient in stabilizing the buoy in descent but, on the contrary, during the rising stage, it tends to behave like a fin which causes a swirling motion as shown in FIG. 2.
The bottom ring makes it possible, in principle, to overcome this drawback by countering this swirling motion since it is placed beneath the center of gravity of the buoy. However, during descent, this bottom ring 13 also behaves like a fin and itself tends to generate a swirling motion. This motion is not exactly the same as that generated, during the rising stage, by the ring 12, because the action of the ballast 14 is not identical to the traction of the cable 11 but, in all, the effects of the two rings impede each other and the overall result is hardly efficient.